There’s not a whole lot to say here. The asm() statement lets you inject assembly language code into your C++ program. This is a huge topic in itself, but outside of this conversation.
The syntax and meaning of the assembly language you insert is dependent on your compiler and platform. The example provided in C++ In a Nutshell is:
asm("mov 4, %eax"); // GNU on Intel IA32
asm("mov eax, 4"); // Borland on Intel IA32
It is important to note that many compilers offer expanded support for asm with code block and other nice features.